I won't even make the argument that all characters are necessary to the party. I won't say that you'll be glad to have that defensive wizard when he is the only one to survive a near tpk or the barbarian when you need to hold a door, or any of the other possibilities. Instead, I'll point out that everyone who plays this game makes a character, usually that they think they'll enjoy, and to talk about how you would leave that character behind or not give him xp just to satisfy your view of "how the game should be played" makes the game less fun for that player.
In that case, I would say your the one who's not playing the game correctly.